remove() during iteration causes items to be skipped

[krabsa]

BPO	1584028
Nosy	@rhettinger

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2006-10-25.01:06:12.000>
created_at = <Date 2006-10-24.21:44:32.000>
labels = ['interpreter-core', 'invalid']
title = 'remove() during iteration causes items to be skipped'
updated_at = <Date 2006-10-25.01:06:12.000>
user = 'https://bugs.python.org/krabsa'

bugs.python.org fields:

activity = <Date 2006-10-25.01:06:12.000>
actor = 'rhettinger'
assignee = 'none'
closed = True
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2006-10-24.21:44:32.000>
creator = 'krabsa'
dependencies = []
files = []
hgrepos = []
issue_num = 1584028
keywords = []
message_count = 2.0
messages = ['30393', '30394']
nosy_count = 2.0
nosy_names = ['rhettinger', 'krabsa']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue1584028'
versions = ['Python 2.4']

[krabsa]

If, when iterating over the contents of a list, the
current item is removed, the next item is skipped.

#Code:
if __name__ == '__main__':
  items = [0,1,2,3,4,5]
  for i in items:
    print i
    items.remove(i)

#End Code

Outputs:
0
2
4

I believe the behavior is undesirable. An argument can
be made to not fix, but the issue is worth noting.

[rhettinger]

Logged In: YES
user_id=80475

Sorry, this isn't a bug -- it is a natural side-effect of
mutating a object while iterating over it. The various
approaches to dealing with this include:

• don't allow mutation while iterating -- dict.iterkeys()
  uses this approach
• iterate over a copy of the object -- dict.keys() uses this
  approach
• iterate over consecutive indices and ignore mutation --
  lists use this approach

Programmers can avoid dealing with this issue by:

• precopying the list:
  for i in items[:]:
  print i
  remove(i)

• building a new list during iteration:
  items[:] = [x for x in items if f(x)]